Monthly Archives: September 2017

If you have spent any time on Beaver Lake in northwestern Arkansas, then you have probably seen sandstone paleokarst features. Some stand tall like towers while others appear to be irregular to rounded masses. It is common to see only the tops of these features when the lake level is low to normal.

Top of sandstone mass in Beaver Lake. Photo taken in October, 2016.

Sandstone mass along Beaver Lake. Photo taken in October, 2016.

These features have been in geology literature since 1858 when David Dale Owen made his first geological reconnaissance of the northern counties. He described a mass of isolated sandstone within adjacent magnesian limestone (now called dolostone) which stands out forming a conspicuous feature in the landscape. Purdue, 1907, called them cave-sandstone deposits and was the first to consider them paleokarst. Purdue and Miser, 1916, noted many of these deposits and concluded several were ancient sinkholes that had been filled with sand. Two theses that pre-date the construction of Beaver Lake (Arrington, 1962, and Staley, 1962) mention numerous sandstone bodies within the Powell. One very large sandstone mass was seen in the White River (Arrington, 1962). It is approximately 45 feet tall! Unfortunately, it is now covered with water.

Sandstone mass in Carroll County. From Owen, 1858

Sandstone mass in the White River near Hwy 12 access to Beaver Lake. From Arrington, 1962.

So how did these features form? First, let’s define paleokarst. Paleokarst consists of karst features that formed in the geologic past and were preserved in the rock record. Karst features include sinkholes, springs, and caves. These features form when acidic rain and ground water dissolves carbonate rocks (mainly rocks that contain calcium carbonate – calcite, or calcium-magnesium carbonate – dolomite).

The majority of sandstone masses are surrounded by dolostone, composed of dolomite, in the Powell Formation. The Powell is Lower Ordovician in age, meaning it formed around 470 million years ago (mya). It is likely that this formation was exposed to weathering at that time. Depressions of various size, called sinkholes, developed on the exposed land surface. Later, sand filled the depressions and eventually became rock called sandstone. The age of the sandstone masses ranges from Middle Ordovician (approx. 450 mya) to Middle Devonian (approx. 390 mya). Therefore, there is a gap in the rock sequence, between dolostone in the Powell and the sandstone, called an unconformity, lasting from 20-80 million years.

Why is paleokarst important, other than being interesting features to observe? Paleokarst provides clues to former geologic conditions and changes in climate and sea level (Palmer and Palmer, 2011). We know that sea level was high in the Lower Ordovician and shallow seas covered all of northern Arkansas. But, in the Middle Ordovician, sea level lowered and the sandstone paleokarst features provide additional evidence supporting this change.

Many sandstone paleokarst features were located while mapping the War Eagle quadrangle. Fifty-two sandstone masses were located around Beaver Lake. This is not a complete list, however, since the main focus of mapping was not a paleokarst inventory.

Above is several pictures of an unidentified plant fossil found in NW Arkansas this past week in the Dye Shale Member of the Bloyd Formation. The fossil is mostly pyrite with an outer coating of calcite (gray crust). It was found in a shale unit and the original plant, or tree, has been squashed by the weight of sediment above it.

At just over 6 feet long and less than an inch thick, it’s an unusually well preserved fossil, especially considering the Dye Shale isn’t known to contain many fossils. It’s also a marine unit and this is certainly a terrestrial plant. Perhaps it was washed in to the environment during a storm and rapidly buried, which led to its preservation. There are no obvious places where branches or leaves might have attached to the trunk and it has a distinct bark pattern that is unlike the well-known plants of the Pennsylvanian Period, such as lycopods, Lepidodendron, or Calamites.

If any fossil savvy readers have a suggestion for its identity, feel free to pass it along. Otherwise, we’ll keep looking into it.